In order to explore the potential impact of cooperative adaptive cruise control (CACC) vehicles on traffic systems, driving comfort is analyzed under different CACC penetrations where manual vehicles, CACC vehicles, and adaptive cruise control (ACC) vehicles are mixed in the process of CACC gradually increasing on roads. The ACC and CACC models validated by PATH program were employed in numerical simulations. Driving comfort of the mixed traffic flow was measured using the international organization for standardization(ISO) 2631-1. Parametric sensitivity analysis on the desired time-gap of ACC and CACC was also conducted. Finally, simulation results were discussed from the perspective of traffic flow stability. It is found that comfort first worsens then upgrades with the increase of CACC penetration. Larger ACC time-gap is apt to restrict the comfort deterioration, while various values of CACC time-gap have little effect on improving the driving comfort. The results indicate that the trend of mixed traffic flow stability affects comfort qualitatively. Vehicle-to-vehicle communication equipment of manual vehicles will benefit the gradual increase of the comfort.